The present invention relates generally to integrated circuit design and fabrication systems. More specifically, the invention relates to mechanisms for inspecting reticles.
Generation of reticles and subsequent optical inspection of such reticles have become standard steps in the production of semiconductors. Initially, circuit designers provide circuit pattern data, which describes a particular integrated circuit (IC) design, to a reticle production system, or reticle writer. The circuit pattern data is typically in the form of a representational layout of the physical layers of the fabricated IC device. The representational layout typically includes a representational layer for each physical layer of the IC device (e.g., gate oxide, polysilicon, metallization, etc.), wherein each representational layer is composed of a plurality of polygons that define a layer""s patterning of the particular IC device.
The reticle writer uses the circuit pattern data to write (e.g., typically, an electron beam writer or laser scanner is used to expose a reticle pattern) a plurality of reticles that will later be used to fabricate the particular IC design. A reticle inspection system may then inspect the reticle for defects that may have occurred during the production of the reticles.
A reticle or photomask is an optical element containing at least transparent and opaque regions, and sometimes semi-transparent and phase shifting regions, as well, which together define the pattern of coplanar features in an electronic device such as an integrated circuit. Reticles are used during photolithography to define specified regions of a semiconductor wafer for etching, ion implantation, or other fabrication process. For many modern integrated circuit designs, an optical reticle""s features are between about 1 and about 5 times larger than the corresponding features on the wafer. For other exposure systems (e.g., x-ray, e-beam, and extreme ultraviolet) a similar range of reduction ratios also apply.
Optical reticles are typically made from a transparent medium such as a borosilicate glass or quartz plate on which is deposited an opaque and/or semi-opaque layer of chromium or other suitable material. However, other mask technologies are employed for direct e-beam exposure (e.g., stencil masks), x-ray exposure (e.g., absorber masks), etc. The reticle pattern may be created by a laser or an e-beam direct write technique, for example, both of which are widely used in the art.
After fabrication of each reticle or group of reticles, each reticle is typically inspected by illuminating it with light emanating from a controlled illuminator. A test image of a portion of the reticle is constructed based on the portion of the light reflected, transmitted, or otherwise directed to a light sensor. Such inspection techniques and apparatus are well known in the art and are embodied in various commercial products such as many of those available from KLA-Tencor Corporation of San Jose, Calif.
During a conventional inspection process, the test image of the reticle is typically compared to a baseline image. Typically, the baseline image is either generated from the circuit pattern data or from an adjacent die on the reticle itself. Either way, the test image features are analyzed and compared with corresponding features of the baseline image. That is, an edge position within the test image is subtracted from a corresponding edge position within the baseline image to calculate a difference value. Each difference value is then compared with a predetermined threshold value. If the test image feature varies from the baseline feature by more than the predetermined threshold, a defect is defined and an error is reported.
An error report for a particular test image will typically only include a list of errors that were detected within the particular test image and corresponding reticle (e.g., the location of each error and a small image of that defect). In other words, the list represents the features within the test image that varied from the baseline image by more than the predetermined threshold. Specifically, the list represents the edge positions within the test image that varied more than the predetermined threshold from the corresponding edge positions of the baseline image, as well as any extra or missing features.
Although conventional inspection techniques provide adequate error data in some applications, this data proves limiting under certain conditions. For example, a user of the reticle may wish to know the actual measured values of particular characteristics of features (i.e., edge position) within the test image as a function of position on the reticle. Additionally, the user may wish to know other measurable values of other characteristics (e.g., line width and corner rounding values). By way of another example, the user may wish to know the amount of variance between the features of the test image and the features of the baseline image as a function of position on the reticle.
Although these variance values may not be large enough to be defined as errors, they may be useful in process control and/or monitoring. Additionally, statistical information of measurable characteristics as a function of position on the reticle, for example, may be used to increase the sensitivity of the inspection process itself, among other applications. That is, the threshold may be adjusted for certain areas of the reticle that typically have more errors than other areas of the reticle. Unfortunately, conventional inspection apparatus and techniques merely provide a list of errors present on the reticle and do not provide any statistical information of measured characteristics of the reticle.
Thus, inspection apparatus and techniques for improving and enhancing information that is output from the inspection procedure are needed. More specifically, inspection mechanisms for providing statistical information about measured characteristics of the reticle are needed.
Accordingly, the present invention addresses the above problems by providing apparatus and methods for providing statistical information during the inspection process. As each feature or region of a test image of a portion of a reticle is evaluated, statistical information is collected for the entire test image. That is, as features of the test image are compared to features of the baseline image, measured characteristic values of the test image (or difference values between the test and baseline images) are collected. The collected measured or difference values may be correlated to a number of reticle parameters, such as a reticle position, a particular area on the reticle, a feature density value of a particular area of the reticle, or a process associated with the reticle under test. A count of the measured characteristic or difference values may also be collected. This collected data (e.g., the count and measured values or difference values) may then be used to compute other statistical parameters, such as standard deviation, minimum, maximum, range (maximum minus minimum), and median or average values.
In one embodiment, a method of inspecting a reticle defining a circuit layer pattern that is used within a corresponding semiconductor process to generate corresponding patterns on a semiconductor wafer is disclosed. A test image of the reticle is provided, and the test image has a plurality of test characteristic values. A baseline image containing an expected pattern of the test image is also provided. The baseline image has a plurality of baseline characteristic values that correspond to the test characteristic values. The test characteristic values are compared to the baseline characteristic values such that a plurality of difference values are calculated for each pair of test and baseline characteristic values. Statistical information is also collected.
In a specific embodiment, the statistical information includes a second plurality of test characteristics values that are of a different type of characteristic than the first plurality of test characteristic values that are compared to the baseline characteristic values. The statistical information may also include a standard deviation value of the second test characteristic values, a median value of the second test characteristic values, and/or an average value of the second test characteristic values. The first test characteristic values may be in the form of edge position values and the second test characteristic values include line width values, corner rounding values, transmission values, gate line width values, contact area values, and/or misalignment values.
In another embodiment, a method of monitoring or adjusting a reticle process that is used to generate reticles is disclosed. The method includes (a) generating a first reticle using a reticle process; (b) providing a test image of the first reticle, wherein the test image has a plurality of test characteristic values; (c) providing a baseline image containing an expected pattern of the test image, wherein the baseline image has a plurality of baseline characteristic values that correspond to a first subgroup of the test characteristic values; (d) comparing the first subgroup of test characteristic values to the corresponding baseline characteristic values such that a plurality of difference values are calculated for each pair of test and baseline characteristic values; (e) collecting statistical information based on a second subgroup of the test characteristic values of the first reticle; and (f) adjusting a process parameter of the reticle process if the statistical information indicates that the second subgroup of test characteristic values deviate from the baseline values by more than a predetermined amount.
In one aspect, the first subgroup is equal to the second subgroup of test characteristic values. In yet another embodiment, the above operations (a) through (e) are repeated for a second reticle. The statistical information for the second reticle is compared to the statistical information for the first reticle, and a process parameter of the reticle process is adjusted if the statistical information for the second reticle varies from the statistical information for the first reticle by more than a second predetermined amount. In yet another embodiment, the process parameter of the reticle process is adjusted so as to reduce variations in the second subgroup of test characteristic values as a function of reticle position.
In another method aspect, a semiconductor process is monitored or adjusted. A reticle defining a circuit layer pattern and statistical information about selected characteristic values of the circuit layer pattern are provided. A circuit layer on a semiconductor wafer is generated using the reticle in a photolithography process. The resulting circuit layer is inspected based at least in part on the statistical information.
In yet another aspect, a computer readable medium containing program instructions for inspecting a reticle defining a circuit layer pattern that is used within a corresponding semiconductor process to generate corresponding patterns on a semiconductor wafer is also disclosed. The computer readable medium includes computer readable code for (i) providing a test image of the reticle, wherein the test image having a plurality of test characteristic values, (ii) providing a baseline image containing an expected pattern of the test image, wherein the baseline image having a plurality of baseline characteristic values that correspond to the test characteristic values, (iii) comparing the test characteristic values to the baseline characteristic values such that a plurality of difference values are calculated for each pair of test and baseline characteristic values, (iv) collecting statistical information, and a computer readable medium for storing the computer readable codes.
In yet another embodiment, a computer readable medium containing program instructions for monitoring or adjusting a semiconductor process is disclosed. The computer readable medium includes computer readable code for providing a reticle defining a circuit layer pattern, computer readable code for providing statistical information about selected characteristic values of the circuit layer pattern, computer readable code for generating a circuit layer on a semiconductor wafer using the reticle in a photolithography process, computer readable code for inspecting the resulting circuit layer based at least in part on the statistical information, and a computer readable medium for storing the computer readable codes.
In another embodiment, a computer readable medium containing program instructions for monitoring or adjusting a reticle process is also disclosed. The computer readable medium includes computer code for (a) generating a first reticle using a reticle process, (b) providing a test image of the first reticle, wherein the test image has a plurality of test characteristic values, (c) providing a baseline image containing an expected pattern of the test image, wherein the baseline image has a plurality of baseline characteristic values that correspond to the test characteristic values, (d) comparing the test characteristic values to the baseline characteristic values such that a plurality of difference values are calculated for each pair of test and baseline characteristic values, (e) collecting statistical information based on the plurality of test characteristic values of the first reticle, and (f) adjusting a process parameter of the reticle process if the statistical information indicates that the test characteristic values deviate from the baseline characteristic values by more than a predetermined amount.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures which illustrate by way of example the principles of the invention.